1. Field of the Invention
The features described herein relate to forming an active channel region using enhanced drop-cast printing techniques for printed electronics. The enhanced drop-cast printing method uses a two-liquid system to achieve the direct growth of well-oriented organic crystals on an active channel region in an organic field-effect transistor (OFET) with high-performance electrical properties, as a result of the proper orientation of molecules in the crystals, the absence of grain boundaries, and low trap densities.
2. Description of the Related Art
Research and development in manufacturing field-effect transistors using organic semiconductors as an active channel layer are being performed, because organic field-effect transistors (OFETs) can be simply and inexpensively manufactured and flexible electronic devices can be manufactured using OFETs (Amanda R. Murphy, et al., Chem. Rev. 2007, 1066). In an attempt to gain a fundamental and rigorous understanding of the charge transport characteristics and electrical properties of crystalline OFETs at a microscopic or molecular level, and to develop printing methods for cost-effective and mass-producible electronic devices, considerable effort has been made in the study of organic crystals composed of π-conjugated organic molecules (J. A. Rogers, et al., Science 2004, 303, 1644).
Generally, compared with networked organic thin-film field-effect transistors (OTFTs), crystalline organic transistors exhibit better electrical properties, such as higher carrier mobility, as a result of the proper orientation of molecules in the crystal, the absence of grain boundaries, and the low trap densities.
However, growing crystals in the highest-mobility direction on an active channel region in OFETs is technically a significant challenge (Z. Bao, et al., Nature 2006, 444, 913). Moreover, in solution-based processes, which are generally favorable for isotropic growth of crystals, it is very difficult to grow a well-oriented organic single crystal in an active channel region in transistors (T. Yamao, et al., Chem. Mater. 2007 19, 3748).
Thus, a practical solution-based approach to place crystalline organic materials in an active channel region in organic transistors requires the development of a direct crystal growth method.
Research has been focused on organic semiconductor materials that can be processed in solution to create printed electronic devices, such as flexible displays, electronic signages, photovoltaic panels, membrane keyboards, radio-frequency identification tags (RFIDs), electronic sensors, integrated electronic circuits, and the like. Printing methods are considered to be cost-effective and suitable for mass production. Functional structures can be made through sequential ejections of constituent materials in a process similar to ink-jet printing. Ink-jet printing methods are the natural and appropriate choice for the production of low-cost printed electronic devices such as OFETs (T. Sekitani, et al., Proc. Natl. Acad. Sci. U.S.A. 2008. 105, 4976).
However, conventional inkjet printing techniques are unable to provide well-oriented organic crystals for an active channel region in OFETs. As a result, carrier mobility is not satisfactory, and thus the OFETs may not be suitable for practical applications. Thus, it would be useful to adapt or improve the conventional inkjet printing method to realize high-performance crystalline organic transistors having well-oriented dozens of organic crystals. The present inventors recognized such drawbacks of the conventional art, which resulted in the conception and development of the various inventive features described hereafter.